This invention relates to the addressing of arrays of electrical elements and concerns a multiplexer circuit for driving such an array. In particular, the invention concerns a row multiplexer for an array of device elements formed with thin-film circuitry. The device elements may be image sensor elements (for example photosensitive diodes) of a large area image sensor, or display elements of a flat panel display (for example a liquid crystal display), or several other types of device element.
Addressing an array of device elements in rows and columns requires each element to be connected to a unique pair of row and column conductors. In the case of an image sensor, a resolution of 300 dpi (dots per inch) may be required so that for an A4 large area image sensor, 2,500 and 3,500 rows and columns are required respectively (with the rows parallel to the long side of the A4 page). Problems therefore arise in forming the interconnections to the large area circuit and the use of multiplexer circuits which are integrated with the large area array circuit is desired, so as to reduce the number of connections to the integrated circuit.
Published European Patent Application EP-A-0633 542 (our reference PHB 33858) discloses such an electronic device comprising an array of device elements which are arranged in rows and columns and which are coupled to row and column conductors. The row conductors are each connected to a multiplexer circuit which comprises a reset line and a select line. The two lines are connected together by a pair of series-connected diodes with a pair of such diodes for each row conductor. The row conductor is connected to the junction of the two diodes, so that when the two diodes associated with a row conductor are in a conducting state, the row conductor is held at a voltage defined by the potential divider action of the two diodes. The diodes are photosensitive diodes which are switched from a blocking state to a conductive state by illumination from a corresponding arrangement of light sources. When EP-A-0 633 542 concerns an image sensor array having photosensitive diodes and switching diodes as the image sensor pixels, the invention described in EP-A-0 633 542 permits the row multiplexer circuits to be fabricated with diodes and using the same technology type as the device elements of the array. However, the row multiplexer circuits require alignment of the light sources with the photosensitive diodes of the multiplexer circuit. Depending on the type of light source used, the switching of the light source may also introduce an undesirable delay in the switching of the multiplexer circuit.